PEDROGAO GRANDE, Portugal, June 19 (Reuters) - More than 1,000 firefighters were still battling Portugal's deadliest forest blaze on Monday after it killed at least 62 people over the weekend.

Prime Minister Antonio Costa, who on Sunday visited Pedrogao Grande, a mountainous area about 200 kilometres (125 miles) northeast of Lisbon, called it the biggest human tragedy in Portugal in living memory.

Welcome light rain that started on Monday morning brought only modest relief to the shocked population and exhausted firefighters. Water planes, including French and Spanish ones, resumed their missions after stopping overnight.

"There is still a lot of forest that can burn and the rain does not make much difference," said Rui Barreto, deputy chief firefighter at the makeshift emergency services headquarters in Pedrogao Grande as thunder rolled through the skies over the ash-covered town.

Firefighters said the weather conditions were still adverse in most areas where the flames were raging. Two army battalions were helping the emergency services.

Dozens of fire engines drove back and forth to fight the raging blaze in areas as far as 20km north of Pedrogao Grande. In a sign of help Portugal is receiving from its European neighbours, four Spanish fire engines were seen driving off from the headquarters.

At least half the victims died in their cars as they tried to flee along a local motorway while many other bodies were found next to the road, suggesting they had probably abandoned their vehicles in panic.

GOVERNMENT ASSURANCES

Despite government assurances that the first response by the emergency services was swift and adequate, many media and residents questioned the efficiency of the operation and the strategic planning in a country which is used to wooded areas burning every year.

"So what failed this Saturday? Everything, as it has failed for decades," read a headline in the daily Publico, which blamed a lack of coordination between services in charge of fire prevention and firefighting and poor forestry reserve planning.

Police said a lightning strike on a tree probably caused the blaze on Saturday in a region hit by an intense heat wave and dry, gusty winds, which fanned the flames.

The regional prosecutor still ordered a criminal investigation into the causes, which he said would be shelved if the police version of events is confirmed. Many forest fires in Portugal are caused by arson or carelessness.

A public petition circulating on the Internet demanding an investigation into possible failures by the authorities has gathered about 270 signatures.

Local residents said they had been without the support of firefighters for hours as their homes burned. Many blamed depopulation of villages that left wooded areas untended. (Writing by Andrei Khalip, editing by Ed Osmond)